1. Field of the Invention
The present invention relates to a chromatic dispersion distribution measurement apparatus (wavelength dispersion distribution measurement apparatus) for measuring a chromatic dispersion distribution (wavelength dispersion distribution) in an optical device to be measured, such as an optical fiber, and a method for the same. Moreover, the present invention relates to a storage medium for storing a program for calculating a chromatic dispersion distribution in an optical device to be measured.
2. Description of Related Art
In recent years, in order to satisfy the demands for high speed information communications, optical communication systems using optical fibers have been constructed. One of the factors in preventing the high speed signal transmission and the long transmission distance in the above optical communication systems, is the chromatic dispersion. The chromatic dispersion is a phenomenon caused by varying the speeds of lights transmitted in a medium, with the wavelength of the light. In the construction of the optical communication systems, it is necessary to grasp the chromatic dispersion characteristic in detail.
A chromatic dispersion distribution measurement apparatus for measuring the chromatic dispersion is shown in, for example, Japanese Patent Application Publication No. Tokukai-Hei 10-83006 (corresponding to the U.S. Pat. No. 5,956,131 and the European Patent Application No. 0819926A2). In the publication, the chromatic dispersion distribution measurement apparatus measures the dispersion distribution in a longitudinal direction of a fiber to be measured, as follows. Two lights having different wavelengths from each other are inputted into the fiber to be measured. A specific wavelength component is extracted by an optical bandpass filter from a four-wave mixed light caused by the interaction between these two lights. A light having the extracted specific wavelength component is inputted into an Optical Time Domain Reflectometer (OTDR).
The four-wave mixing (FWM) is a phenomenon caused by the non-linearity of a plurality of lights having different wavelengths from each other in an optical fiber. For example, when two lights have wavelengths xcex1 and xcex2 respectively, a wavelength xcex3 of a light (Stokes light) caused by this phenomenon and a wavelength xcex4 of a light (anti-Stokes light) caused by the phenomenon satisfy the following equation (1).
xcex2xe2x88x92xcex1=xcex1xe2x88x92xcex4=xcex3xe2x88x92xcex2xe2x80x83xe2x80x83(1)
The chromatic dispersion distributions which are measured by using the OTDR generally vary with each optical fiber to be measured. Moreover, a chromatic dispersion value is marked with a constant sign (positive (+) or negative (xe2x88x92) sign) (hereinbelow, referred to as xe2x80x9csignxe2x80x9d) in spite of wavelengths of the inputted lights. That is, the sign of a chromatic dispersion value depends on an optical fiber to be measured. By using a former chromatic dispersion distribution measurement apparatus, only an absolute value of a chromatic dispersion value can be measured. The sign of a chromatic dispersion value is judged by using another apparatus.
With reference to FIGS. 1 and 7, a chromatic dispersion distribution in a long distance optical cable, which is measured by a former chromatic dispersion distribution measurement apparatus, is explained.
FIG. 1 is a view showing a schematic structure of a long distance optical cable 200 as an optical device to be measured. FIG. 7 shows a chromatic dispersion distribution in the long distance optical cable 200, which is shown without marking each chromatic dispersion value with the sign. The chromatic dispersion distribution shown without marking each chromatic dispersion value with the sign, is measured by a chromatic dispersion distribution measurement apparatus. The chromatic dispersion distribution shown without marking each chromatic dispersion value with the sign, is also measured as an interim data by an optical fiber chromatic dispersion distribution measurement apparatus 100 to which the present invention is applied. The distribution will be explained in detail later.
The long distance optical cable 200 is constructed by jointing three optical fibers H1, H2 and H3. A range of the optical fiber H1 in the longitudinal direction of the long distance optical cable 200 is 0 to 20 km. A range of the optical fiber H2 is 20 to 40 km. A range of the optical fiber H3 is 40 to 60 km. The chromatic dispersion value marked with the sign (hereinbelow, referred to as xe2x80x9csigned chromatic dispersion valuexe2x80x9d), of the optical fiber H1 is +17 (ps/nm/km). The signed chromatic dispersion value of the optical fiber H2 is xe2x88x924 (ps/nm/km). The signed chromatic dispersion value of the optical fiber H3 is xe2x88x9250 (ps/nm/km).
As shown in FIG. 7, a chromatic dispersion distribution in the long distance optical cable 200, which is measured by a former chromatic dispersion distribution measurement apparatus is expressed by using each absolute value of the chromatic dispersion value of the optical fibers H1 to H3. In detail, the absolute value of the chromatic dispersion value of the optical fiber H1, which is denoted by B1 is xe2x80x9c17 (ps/nm/km)xe2x80x9d. The absolute value of the chromatic dispersion value of the optical fiber H2, which is denoted by B2 is xe2x80x9c4 (ps/nm/km)xe2x80x9d. The absolute value of the chromatic dispersion value of the optical fiber H3, which is denoted by B3 is xe2x80x9c50 (ps/nm/km)xe2x80x9d.
However, there was a problem as explained below in such a former chromatic dispersion distribution measurement apparatus. The chromatic dispersion distribution in a long distance optical cable constructed by jointing a plurality of optical fibers, which is measured by a former chromatic dispersion distribution measurement apparatus, is expressed by using each absolute value of the chromatic dispersion value of the optical fiber. Moreover, each chromatic dispersion value of this chromatic dispersion distribution is outputted as a positive (+) value in spite of the optical fibers. Therefore, it is difficult to measure correctly a chromatic dispersion distribution in the long distance optical cable and an accumulated chromatic dispersion value of the long distance optical cable, in consideration of each sign of the chromatic dispersion value.
For example, when the accumulated chromatic dispersion value of the long distance optical cable 200 is calculated in accordance with the measurement result shown in FIG. 7, which is measured by a former chromatic dispersion distribution measurement apparatus, the calculating result is the sum of the products of each absolute value of the chromatic dispersion value and the length of each optical fiber. That is, the accumulated chromatic dispersion value is xe2x80x9c17xc3x9720+4xc3x9720+5xc3x9720=1420 (ps/nm)xe2x80x9d. However, the correct accumulated chromatic dispersion value of the long distance optical cable 200 (the sum of the products of each signed chromatic dispersion value and the length of each optical fiber) is xe2x80x9c17xc3x9720+(xe2x88x924)xc3x9720+(xe2x88x9250)xc3x9720=xe2x88x92740 (ps/nm)xe2x80x9d. Therefore, the calculated accumulated chromatic dispersion value is different from the correct accumulated chromatic dispersion value.
In order to solve the above-described problems, an object of the present invention is to provide a chromatic dispersion distribution measurement apparatus which can measure a chromatic dispersion value marked with a positive sign or a negative sign, and to provide a measurement method which can measure a chromatic dispersion value marked with a positive sign or a negative sign. Moreover, another object is to provide a storage medium storing a program which can calculate a chromatic dispersion value marked with a positive sign or a negative.
That is, in accordance with a first aspect of the present invention, a chromatic dispersion distribution measurement apparatus for measuring a chromatic dispersion distribution in an optical device to be measured, comprises:
two light sources for emitting two lights having different wavelengths from each other;
an intensity measurement unit for measuring an intensity of a four-wave mixing light outputted from the optical device to be measured, as a function of a transmission distance of the four-wave mixing light, when the two lights are simultaneously inputted into the optical device to be measured;
a calculating unit for calculating a chromatic dispersion distribution in the optical device to be measured, in accordance with the intensity of the four-wave mixing light as the function of the transmission distance, which is measured by the intensity measurement unit;
a portion specifying unit for specifying a portion of the optical device to be measured in a longitudinal direction thereof;
a sign setting unit for setting a correct sign selected out of a positive sign and a negative sign, which is to be marked on a chromatic dispersion value in the specified portion of the optical device to be measured; and
a sign converting unit for converting an initial sign marked on the chromatic dispersion value in the specified portion, which is extracted from the chromatic dispersion distribution calculated by the calculating unit, into the correct sign which is set by the sign setting unit.
In accordance with a second aspect of the present invention, a chromatic dispersion distribution measurement method for measuring a chromatic dispersion distribution in an optical device to be measured, comprises:
emitting two lights which have different wavelengths from each other;
measuring an intensity of a four-wave mixing light outputted from the optical device to be measured, as a function of a transmission distance of the four-wave mixing light, when the two lights are simultaneously inputted into the optical device to be measured;
calculating a chromatic dispersion distribution in the optical device to be measured, in accordance with the measured intensity of the four-wave mixing light as the function of the transmission distance;
specifying a portion of the optical device to be measured in a longitudinal direction thereof;
setting a correct sign selected out of a positive sign and a negative sign, which is to be marked on a chromatic dispersion value in the specified portion of the optical device to be measured; and
converting an initial sign which is marked on the chromatic dispersion value in the specified portion, which is extracted from the calculated chromatic dispersion distribution, into the set correct sign.
Hereinafter, the term xe2x80x9csignxe2x80x9d generally means one of a positive sign and a negative sign. Moreover, the initial sign of an absolute value is treated as a positive sign.
According to the first and second aspects of the present invention, in the case that the optical device to be measured is constructed by jointing a plurality of optical device portions having chromatic dispersion values which are different from each other, when the correct sign to be marked on each chromatic dispersion value of the optical device portions and each joint position of the optical device portions are known to an operator, the chromatic dispersion value marked with the correct sign, of the optional optical device portion constituting the optical device to be measured, can be easily calculated without individually measuring each chromatic dispersion value of the optical device portions. Therefore, the chromatic dispersion distribution measurement apparatus and method having the enhanced functionality and convenience, can be realized.
The calculating unit may calculate a signed chromatic dispersion distribution expressed by marking each chromatic dispersion value with the correct sign, in the optical device to be measured, in accordance with the chromatic dispersion value which is marked with the correct sign into witch the initial sign is converted by the sign converting unit.
The calculating may be carried out by calculating a signed chromatic dispersion distribution expressed by marking each chromatic dispersion value with the correct sign, in the optical device to be measured, in accordance with the chromatic dispersion value which is marked with the correct sign.
Hereinafter, the signed chromatic dispersion distribution is the chromatic dispersion distribution expressed by marking each chromatic dispersion value with the correct sign.
In the case that the optical device to be measured is constructed by jointing a plurality of optical device portions having chromatic dispersion values which are different from each other, when the correct sign to be marked on each chromatic dispersion value of the optical device portions and each joint position of the optical device portions are known to the operator, the signed chromatic dispersion distribution in the optical device to be measured, on which the sign of the chromatic dispersion value of each optical device portion is correctly reflected, can be easily calculated. Therefore, the chromatic dispersion distribution measurement apparatus and method having the much enhanced functionality and convenience, can be realized.
The calculating unit may calculate an accumulated chromatic dispersion value of the optical device to be measured, in accordance with the calculated signed chromatic dispersion distribution.
The calculating may be carried out by calculating an accumulated chromatic dispersion value of the optical device to be measured, in accordance with the calculated signed chromatic dispersion distribution.
Therefore, in the case that the optical device to be measured is constructed by jointing a plurality of optical device portions having chromatic dispersion values which are different from each other, when the correct sign to be marked on each chromatic dispersion value of the optical device portions and each joint position of the optical device portions are known to the operator, the correct accumulated chromatic dispersion value of the optical device to be measured, can be easily calculated without individually measuring each chromatic dispersion value of the optical device portions. Therefore, the chromatic dispersion distribution measurement apparatus and method having the enhanced functionality and convenience, can be realized. Further, an optical cable which enables a long distance communication can be easily constructed by suitably combining and jointing optical device portions having chromatic dispersion values which are different from each other.
The chromatic dispersion distribution measurement apparatus may further comprise:
a directional coupler unit for outputting the two lights to the optical device to be measured, and for outputting a reflected four-wave mixing light which is reflected from the optical device to be measured, to the intensity measurement unit; wherein
the intensity measurement unit measures the intensity of the reflected four-wave mixing light as the function of the transmission distance.
In accordance with a third aspect of the present invention, a chromatic dispersion distribution measurement apparatus, comprises:
a portion information obtaining unit for obtaining a portion information which specifies a portion of an optical device to be measured;
a sign information obtaining unit for obtaining a sign information which indicates a correct sign to be marked on a chromatic dispersion value in the specified portion; and
a sign converting unit for converting an initial sign marked on the chromatic dispersion value in the specified portion, into the correct sign in accordance with the sign information obtained by the sign information obtaining unit.
The chromatic dispersion distribution measurement apparatus may further comprise: a calculating unit for calculating a signed chromatic dispersion distribution expressed by marking each chromatic dispersion value with the correct sign, in the optical device to be measured, in accordance with the chromatic dispersion value marked with the correct sign into which the initial sign is converted by the sign converting unit.
The calculating unit may calculate an accumulated chromatic dispersion value of the optical device to be measured, in accordance with the calculated signed chromatic dispersion distribution.
In accordance with a fourth aspect of the present invention, a chromatic dispersion distribution measurement method, comprises:
obtaining a portion information which specifies a portion of an optical device to be measured;
obtaining a sign information which indicates a correct sign to be marked on a chromatic dispersion value in the specified portion; and
converting an initial sign marked on the chromatic dispersion value in the specified portion, into the correct sign in accordance with the obtained sign information.
The chromatic dispersion distribution measurement method may further comprise: calculating a signed chromatic dispersion distribution expressed by marking each chromatic dispersion value with the correct sign, in the optical device to be measured, in accordance with the chromatic dispersion value marked with the correct sign.
The chromatic dispersion distribution measurement method may further comprise: calculating an accumulated chromatic dispersion value of the optical device to be measured, in accordance with the calculated signed chromatic dispersion distribution.
In accordance with a fifth aspect of the present invention, a storage medium has a program recorded thereon, which is executable by a computer, wherein the program comprises:
a first program code for obtaining a portion information which specifies a portion of an optical device to be measured;
a second program code for obtaining a sign information which indicates a correct sign to be marked on a chromatic dispersion value in the specified portion; and
a third program code for converting an initial sign marked on the chromatic dispersion value in the specified portion, into the correct sign in accordance with the obtained sign information.
The program may further comprise: a fourth program code for calculating a signed chromatic dispersion distribution expressed by marking each chromatic dispersion value with the correct sign, in the optical device to be measured, in accordance with the chromatic dispersion value marked with the correct sign.
The program may further comprise: a fifth program code for calculating an accumulated chromatic dispersion value of the optical device to be measured, in accordance with the calculated signed chromatic dispersion distribution.